Hemoglobin (Hb) A1c is a stable Amadori product formed through Amadori rearrangement of a Schiff base which is nonenzymatically produced between the amino group of β-chain N-terminus valine and the aldehyde group of glucose. It is also classified as a glycated protein having a structure formed of a valine residue and fructose bonded thereto. Clinically, HbA1c is correlated with a mean blood sugar level of past one to two months, and therefore, HbA1c serves as an important indicator in control of diabetes. Thus, there still exists demand for a quantitative HbA1c assay method which is rapid, convenient, accurate, and practical.
As a practical standard assay methodology for HbA1c, IFCC (International Federation of Clinical Chemistry and Laboratory Medicine) adopts a method which includes separation, by HPLC, of a β-N-terminal 6-peptide fragment which is likely to have fructosyl valine and is obtained through hydrolysis of hemoglobin with endoprotease Glu-C and quantitation of the separated fragment through capillary electrophoresis or mass spectrometry (Kobold U., et al; Candidate Reference Methods for Hemoglobin A1c Based on Peptide Mapping; Clin. Chem., 43, 1944-1951 (1997)). However, this method requires a special apparatus and entails cumbersome maneuvers and poor economy, making this method impractical.
Existing methods for measuring HbA1c which are currently employed in practice include HPLC employing, as a carrier, a special hard gel having a hydrophobic group or a cation exchange group, and latex immunoagglutination employing anti-HbA1c antibody. These existing methods, requiring expensive instruments or multi-step immunological reactions, are not necessarily satisfactory in terms of speed, convenience, or accuracy.
In recent years, there have been reported enzyme-based assay method for glycated proteins, such as HbA1c and glycated albumin, which include degradation of glycated protein with protease, and employment of fructosyl amino acid oxidase (FAOD) or a similar enzyme that reacts on a glycated amino acid (Japanese Patent Application Laid-Open (kokai) Nos. H05-192193, H07-289253, H08-154672, H06-046846, and H08-336386, WO97/13872, WO02/06519, Japanese Patent Application Laid-Open (kokai) No. 2001-054398).
In any of these methods, in order to avoid difficulty encountered by FAOD or a similar enzyme in acting on glycated protein, regardless of the glycated protein being HbA1c or glycated albumin, glycated amino acids (fructosyl valine for HbA1c; fructosyl lysine for glycated albumin) which are characteristic to respective glycated proteins are cut out from a glycated peptide or glycated protein, and the obtained glycated amino acids are used as substrates for FAOD, etc. Therefore, glycated amino acids for such purposes have to be cut out effectively so that they can serve as substrate for FAOD, etc.
To achieve the above object, research efforts have been undertaken to search for a protease which enables glycated amino acids to be effectively cut out from glycated protein, and heretofore, numerous proteases have been reported. However, no report has disclosed information about the method of cutting out the glycated amino acid (or a peptide containing the glycated amino acid) from a glycated protein, or, in more specifically, the length of the peptide chain cut out from the glycated protein. From this viewpoint, therefore, it remains unknown as to whether or not the disclosures of the above publications are in fact practical.
Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 2001-95598 discloses a method for measuring glycated protein, in which a sample is treated with protease, and the resultant free-form glycated peptide is reacted with glycated peptide oxidase. In the disclosed method, however, there still remains an unsolved problem in that, since the glycated peptide oxidase substantially acts on fructosyl dipeptide, the method is not effective for a fructosyl peptide, which is longer than fructosyl dipeptide, and similar to the case of the conventional approach of using FAOD or a similar substance, a fructosyl dipeptide capable of serving as a substrate must be cut out effectively.
In another report, FAOD is used in combination with another enzyme (Japanese Patent Application Laid-Open (kokai) No. 2000-333696). However, the disclosed method is directed to improvement in measurement sensitivity by measuring hydrogen peroxide from two different sources; i.e., hydrogen peroxide generated from reaction between FAOD and glycated amino acid cut out with protease, and the other hydrogen peroxide generated from reaction between glucosone, which is a concurrently generated degradation product of glycated amino acid, and glucose oxidase. Thus, the method does not intend to perform defructosylation of glycated peptides of different lengths.